1. Field of the Invention
The present invention relates to electronic display devices. More particularly, the present invention relates to an inverter circuit, a backlight assembly, and a liquid crystal display with the backlight assembly.
2. Description of the Related Art
Recently, information processing devices have rapidly evolved to encompass a wide variety of physical configurations and functionalities. Information processed by these processing devices takes the form of an electrical signal. Therefore, users require a display device to visually recognize information processed by the information processing devices.
One example of an existing display device is a flat panel display such as a liquid crystal display (“LCD”). An LCD displays an image using liquid crystals. Relative to other display devices, an LCD is thin, lightweight, consumes little power, and utilizes a low driving voltage. Therefore, an LCD is widely used in various fields.
Such an LCD includes a liquid crystal panel displaying an image and a backlight assembly providing light to the liquid crystal panel. (An illustrative example of an LCD panel is disclosed, for example, in Japanese Patent Publication No. 2005-49747).
FIG. 9 is a circuit diagram of a conventional backlight assembly. FIG. 10 illustrates an exemplary arrangement for the conventional backlight assembly.
Referring to FIG. 9, the conventional backlight assembly includes twenty-four cold cathode fluorescent lamps (“CCFLs”) 910 and twenty-four balance transformers 920a-920x. As a liquid crystal panel increases in size, the backlight assembly may be equipped with a plurality of CCFLs to provide uniform brightness in the liquid crystal panel.
Sinusoidal voltages are applied from an inverter 900 to the CCFLs 910, and thus sinusoidal currents flow through the CCFLs 910. If sinusoidal voltages with the same polarity are applied to respective first terminals of the CCFLs 910, interference with a driving circuit of the liquid crystal panel occurs to generate interference pattern noise on the liquid crystal panel. Additionally, in the case of a large-sized LCD utilizing CCFLs 910 having a long length, when the CCFLs 910 are driven by an one-side-high, voltage-driving method, it is virtually impossible to maintain uniform brightness in a longitudinal direction along the CCFLs 910. To prevent these problems, the CCFLs 910 are divided into two groups as illustrated in FIG. 9, and high sinusoidal voltages with opposite polarities are applied, respectively, to the two groups. That is, the inverter 900 is configured to output both a positive high voltage (“PHV”) and a negative high voltage (“NHV”). The positive high voltage/negative high voltage is applied to the left side/right side, respectively, of the odd-numbered CCFLs 910 (when numbered from the top), and to the right sides/left sides, respectively, of the even-numbered CCFLs 910.
The CCFLs 910 have a negative resistance and are all connected in parallel to each another. Therefore, when a current starts to flow through a given one of the CCFLs 910, the resistance of this CCFL decreases and thus a current easily flows through this CCFL. Since current is concentrated at this CCFL, the remaining CCFLs are not turned on. To prevent this problem, the balance transformers 920a-920x are connected in series to the CCFLs 910, as illustrated in FIG. 9.
The balance transformers 920a-920l are disposed at the left sides of the CCFLs 910, while the balance transformers 920m-920x are disposed at the right sides of the CCFLs 910. The balance transformers 920a˜920x include primary coils 921a-921x connected directly to the CCFLs 910, respectively, and secondary coils 922a-922b installed adjacent to the primary coils 921a˜921x, respectively. When a current flows through the CCFLs 910, a current flows through the primary coils 921a-921x, and a current also flows through the adjacent secondary coils 922a-922x. Since the secondary coils 922a-922x are connected in series to form a loop, the current flowing through the secondary coils 922a-922x causes the current to flow through the primary coils 921a-921x. As a result, currents flowing through the CCFLs 910 become substantially equal to one another.
In this configuration, a balancing voltage of each balance transformer necessary for balancing the CCFLs 910 can be obtained by grounding one point of the secondary coils 922a-922x and detecting a voltage between the grounded point and a detection node 940 remote from the grounded point. In a normal state, the balancing voltage is in the range of about 1 V to about 2 V.
This balancing voltage varies with the distribution of the resistances including the negative resistances of the CCFLs 910. Active use of this property enables detection of an open circuit or short circuit attributable to a failure in the CCFLs 910. That is, when an open circuit or short circuit occurs due to a failure in the CCFLs 910, a voltage (e.g., 5˜6 V) higher than a normal voltage is detected at the detection node 940 as a result of the balancing operations of the balance transformers 920a-920x. 
The conventional backlight assembly has two problems. One is lifetime degradation of the CCFLs 910, and another is that a temperature gradient makes it difficult to troubleshoot a failure in the CCFLs 910. These problems will now be described in greater detail with reference to FIGS. 9 and 10.
Referring to FIG. 9, the negative high voltage (“NHV”) is directly applied to the CCFLs 910, while the positive high voltage (“PHV”) is indirectly applied to the CCFLs 910 through the balance transformers 920a-920x. Thus, there is a difference between loading of the negative and positive high voltages NHV and PHV. In general, since the high voltage output uses virtually identical driving pulses with different polarities in the same circuit, an imbalance may occur in positive and negative driving pulses when there is a difference in loading. When an imbalance occurs in the driving pulses, the lifetime of the CCFLs 910 is shortened due to migration of mercury vapor therein.
Referring to FIG. 10, in the conventional backlight assembly, the CCFLs 910 are disposed horizontally in a vertically-standing protection structure 1020. The protection structure 1020 has a rear surface covered with a reflection plate 1010 and a front surface covered with a diffusion plate 1000. In the conventional backlight assembly, temperature increases in an upward direction due to heat by light emitted from the CCFLs 910, resulting in a temperature gradient.
The CCFLs 910 each have a temperature-dependent resistance. Therefore, due to the temperature gradient, the upper CCFLs 910 have a lower resistance while the lower CCFLs 910 have a higher resistance. To eliminate the resistance differential between the CCFLs 910, the balance transformers 920a-920x operate to balance the CCFLs 910. Accordingly, a voltage of, for example, about 3V is induced at the detection node 940. When an increase in voltage is detected at the detection node 940 in the conventional backlight assembly, it is virtually impossible to find out which of the resistance differences between the CCFLs 910, and an open or short circuit due to failure in a CCFL 910, has caused the voltage increase. Accordingly, it is difficult to accurately troubleshoot failure in the CCFL 910.